1. Field of the Invention
This invention relates to end-functionalized polymers, processes for making the same, and polymers made using such end-functionalized polymers. More particularly, the invention relates to a controlled free-radical polymerization process for forming end-functionalized polymers, particularly by a degenerative iodine transfer (DIT) and atom transfer radical polymerization (ATRP) processes. The resultant end-functionalized polymers have a high degree of functionality, a polydispersity less than 2.5, and a predetermined molecular weight. The resultant end-functionalized polymers are useful as reactive intermediates in condensation polymerization, chain polymerization and heterogeneous polymerization reactions.
2. Description of the Prior Art
Controlled free-radical polymerization processes, including ATRP and DIT, are prior art processes for free-radical polymerization. In degenerative iodine transfer polymerization, chain growth is controlled by iodine atoms, which reversibly react with the growing polymer chain ends thereby, limiting side reactions. Iodine atoms are introduced into the reaction using iodine transfer reagents, and polymer radicals are initially generated with a small amount of a conventional initiator.
The atom transfer radical polymerization process can also produce products with more uniform and more highly controlled architecture. The process includes free-radical polymerization of one or more monomers, in the presence of an initiator having a transferable atom or group, and a transition metal compound with an appropriate ligand. The transition metal compound has the formula ML.sub.n, the ligand L being any N-,O-,P-, or S- containing compound, which can coordinate to the transition metal through a .sigma.-bond or any carbon-containing compound which can coordinate through a .pi.-bond, such that direct bonds between the transition metal in growing polymer radicals are not formed. The formed copolymer is then isolated.
Application of the degenerative transfer process in the production of polymers is disclosed in the following references: Japanese Kokai No. 4-132706 (1992), assigned to Nippon Shokubai, discloses a DIT process for the production of telechelic polymers having hydroxyl groups at the ends. The initial formula of the reagent used is X--R--X' wherein X is bromine or iodine and R is a bivalent C1-C8 hydrocarbon. The reagents used in the method are not efficient, and thus require a great excess of the iodo reagents (0.01-10 moles monomer per mol of the reagent) to produce polymers having a molecular weight of 1500 and greater. Further, the molar ratio of halide reagent to conventional initiator is extremely high, being on the order of 50 to 500 to 1. The functionalization process for converting the chain-end iodides to a hydroxyl group is also inefficient. In this regard, four reactions are specified: (1) hydrolysis; (2) substitution with diols; (3) substitution with hydroxy amines; and (4) substitution with carboxylates. Reactions 1 and 2 promote side reaction with ester containing polymers whereas reactions 3 and 4 are often slow and incomplete. The molecular weights obtained by the method disclosed in Nippon Shokubai Japanese Kokai No. 4-132706 are for the most part high, that is, in excess of 5000.
U.S. Pat. No. 5,439,980 issued in 1995 to Daikin Industries discloses a DIT process wherein block copolymers are synthesized using an iodine reagent and two monomers, which are added simultaneously. The process relies on large reactivity differences between the monomers, and introduces no functional endgroups.
U.S. Pat. No. 5,455,319 issued in 1995 to Geon describes the use of DIT to produce vinyl chloride homopolymers and some random copolymers of vinyl chloride. The iodine transfer reagents employed in the '319 patent are efficient in that they are activated reagents. But the DIT polymerization process in an aqueous media is described only for vinyl chloride polymers and the patent does not address end-functional polymers.
K. Matyjaszewsky, in Macromolecules, Vol. 28, pages 2093-2095 and 8051-8056 (1995) describes a process for controlled polymerization using iodine compounds. Neither efficient difunctional transfer agents nor reagents having an incorporated functional group are disclosed.
Atom transfer radical polymerization (ATRP), on the other hand, is also described in the prior art. For example, WO 96/304212 to Matyjaszewski and Carnegie-Mellon University describes metal catalyzed free-radical polymerization using an alkyl halide initiator to control the polymerization.
The general idea of using a functionalized initiator for ATRP or functionalizing the halide end group from an ATRP polymer is mentioned in J-S Wang, D. Grezsta, K. Matyjaszewski, Polym. Mater. Sci. Eng., 73, 416 (1995). No examples are provided in the article, nor is it obvious how to carry out the hypothesis.
The synthesis of a polymer with an allyl end group using an allyl initiator or substitution with allyl trimethylsilane, and the synthesis of polystyrene with one amine end group using a trimethylsilyl azide reaction followed by hydrolysis are described in Y. Nakagawa, S. Gaynor, K. Matyjaszewaski, Polym. Prep., Am. Chem. Soc., Polym. Div., 37(1), 577 (1996).
A polymer with a vinyl acetate group formed using a functionalized initiator is described in K. L. Beers, S. G. Gaynor, K. Matyjaszewski, Polym. Prep., Am. Chem. Soc., Polym. Div., 37(1), 571 (1996).
Hydroxy end-functionalized polymers, and processes for making the same using non-living free-radical polymerizations, are also disclosed in prior art, European Patent No. EP 0622378A1 to Goldschmidt A G. This patent describes polymethacrylate diols and a process for making the same. The process is a conventional free-radical polymerization process initiated in the presence of a large amount of mercaptoethanol chain transfer agent. The polymer chain starts from mercaptoethanol and terminates with the methacrylate group, which is then converted to a hydroxyl containing moiety by a selective substitution reaction using an aliphatic diol in the presence of Ti(OR).sub.4. The chain end substitution reaction specified is a moisture-sensitive and costly process. Furthermore, the reaction is only selective and efficient for methyl methacrylate polymers thus limiting the general applicability.
U.S. Pat. No. 5,391,655 issued in 1995 to Nippon Shokubai describes a process wherein vinyl monomers are polymerized by conventional free-radical polymerization in the presence of a great excess of a disulfide reagent containing two hydroxyl groups at each end. The formula of the disulfide reagent is HO--R--S--S--R'--OH and the molar concentration of disulfide reagent is greater than 50 times that of the initiator and at least 0.5 of the vinyl monomer. The process is flawed in that it cannot produce pure difunctional telechelics and in that large amounts of the functionalization reagents are needed.
Thus, there exists a need for a process capable of providing an end-functionalized polymers having a predictable molecular weight, high degree of functionality, and low polydispersity. The process must be sufficiently flexible to control molecular weight as well as polymer architecture. A living or controlled free-radical process followed by an efficient functionalization step provides a solution and is presented herein. Efficient iodine transfer agents or bromide initiators and inexpensive functionalization reagents are also needed.
The resultant end-functionalized polymers are useful as reactive intermediates for condensation polymerization of polyurethanes, polyesters and epoxides; chain polymerization to form graft copolymers and crosslinked copolymers; and polymeric emulsifiers.